U.S. patent application number 16/208193 was filed with the patent office on 2019-06-13 for x-ray tomography inspection systems and methods.
The applicant listed for this patent is Rapiscan Systems, Inc.. Invention is credited to Edward James Morton.
Application Number | 20190178821 16/208193 |
Document ID | / |
Family ID | 66696063 |
Filed Date | 2019-06-13 |
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United States Patent
Application |
20190178821 |
Kind Code |
A1 |
Morton; Edward James |
June 13, 2019 |
X-Ray Tomography Inspection Systems and Methods
Abstract
An optical assembly for use in an X-ray inspection system. The
optical assembly has a light source, a photocathode positioned such
that it is in a path of light emitted by the light source, and at
least two dynodes. One of the dynodes is positioned to receive
electrons emitted by the photocathode and the other dynode is
positioned to receive electrons emitted by the first dynode. The
light source is preferably one of an LED light source or a LASER
light source.
Inventors: |
Morton; Edward James;
(Guildford, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Rapiscan Systems, Inc. |
Torrance |
CA |
US |
|
|
Family ID: |
66696063 |
Appl. No.: |
16/208193 |
Filed: |
December 3, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62597155 |
Dec 11, 2017 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61B 6/03 20130101; G01N
23/083 20130101; H01J 35/06 20130101; H01J 35/08 20130101; G01V
5/0008 20130101; H01J 35/14 20130101; H01J 35/064 20190501; H01J
2235/062 20130101 |
International
Class: |
G01N 23/083 20060101
G01N023/083; A61B 6/03 20060101 A61B006/03; H01J 35/08 20060101
H01J035/08; H01J 35/06 20060101 H01J035/06 |
Claims
1. An optical device configured for use in an X-ray inspection
system, the optical device comprising: a light source configured to
emit light; a photocathode proximate the light source, positioned
such that it is in a path of the light emitted by the light source,
and configured to emit a first plurality of electrons; a first
dynode positioned to receive the first plurality of electrons
emitted by the photocathode and configured to emit a second
plurality of electrons in response to receiving the first plurality
of electrons; and a second dynode positioned to receive the second
plurality of electrons emitted by the first dynode and configured
to emit a third plurality of electrons in response to receiving the
second plurality of electrons.
2. The optical device of claim 1 wherein at least a portion of the
optical device is enclosed in a vacuum sealed housing and wherein
the vacuum sealed housing comprises at least one of glass or
metal.
3. The optical device of claim 2 wherein the light source is
positioned outside the housing.
4. The optical device of claim 2, wherein the photocathode
comprises a material deposited over an optically transparent glass
within the vacuum sealed housing.
5. The optical device of claim 1 wherein the optical device further
comprises at least one of a grid electrode or a focus
electrode.
6. The optical device of claim 1 wherein the light source is a
light emitting diode (LED).
7. The optical device of claim 6 wherein the LED emits at least one
of blue light or white light.
8. The optical device of claim 1 wherein the light source a
LASER.
9. The optical device of claim 2 wherein the photocathode, the
first dynode, and the second dynode are placed within the vacuum
sealed housing.
10. An X-ray inspection system comprising: a stationary X-ray
source extending around a scanning volume, wherein the stationary
X-ray source comprises: a plurality of source points, wherein each
of the plurality of source points is configured to generate X-rays
and direct the X-rays into the scanning volume and wherein each of
the plurality of source points comprises: a light assembly
comprising: a light source configured to emit light; a photocathode
proximate the light source, positioned such that it is in a path of
the light emitted by the light source, and configured to emit a
first plurality of electrons; a first dynode positioned to receive
the first plurality of electrons emitted by the photocathode and
configured to emit a second plurality of electrons in response to
receiving the first plurality of electrons; and a second dynode
positioned to receive the second plurality of electrons emitted by
the first dynode and configured to emit a third plurality of
electrons in response to receiving the second plurality of
electrons; and an anode assembly positioned to receive the third
plurality of electrons and configured to convert the third
plurality of electrons to said X-rays; an X-ray detector array
extending around the scanning volume and arranged to detect X-rays
from the anode assembly which have passed through the scanning
volume; a conveyor arranged to convey the items through the
scanning volume; and at least one processor for processing the
detected X-rays to produce images of items passing through the
scanning volume.
11. The X-ray inspection system of claim 10 wherein the anode
assembly emits the X-rays from a plurality of different emission
points.
12. The X-ray inspection system of claim 10 wherein each of the
plurality of source points is enclosed in a vacuum sealed housing
and wherein the housing comprises at least one of glass or
metal.
13. The X-ray inspection system of claim 10 wherein the light
assembly further comprises at least one of a grid electrode or a
focus electrode.
14. The X-ray inspection system of claim 10 wherein the
photocathode comprises a material deposited over an optically
transparent glass.
15. The X-ray inspection system of claim 10 wherein the light
source is a light emitting diode (LED).
16. The X-ray inspection system of claim 10 wherein the stationary
X-ray source comprises the plurality of source points and the anode
assembly positioned in an enclosed housing.
17. The X-ray inspection system of claim 15 wherein the LED emits
one of blue light or white light.
18. The X-ray inspection system of claim 10 wherein the
photocathode and at least one of the first dynode or the second
dynode are placed inside a vacuum sealed housing.
19. A method for scanning items using an X-ray inspection system,
the method comprising: passing an item to be scanned through an
enclosed inspection volume; emitting X-rays from a stationary X-ray
source positioned around the inspection volume by: illuminating a
photocathode from a light source that emits light towards the
photocathode; receiving, at a first dynode, a first plurality of
electrons emitted by the photocathode; receiving, at a second
dynode, a second plurality of electrons emitted by the first
dynode; receiving, at an anode assembly, a third plurality of
electrons emitted by the second dynode and converting the third
plurality of electrons to said X-rays; detecting X-rays from the
stationary X-ray source which have passed through the enclosed
inspection volume; and processing the detected X-rays to produce
scanning images of the items.
20. The method of claim 19, wherein a number of the second
plurality of electrons is greater than a number of the first
plurality of electrons.
21. The method of claim 20, wherein a number of the third plurality
of electrons is greater than a number of the second plurality of
electrons.
22. The method of claim 19 wherein at least one of the
photocathode, first dynode, or second dynode is enclosed in a
vacuum sealed glass or metal housing.
23. The method of claim 19 wherein the photocathode is deposited on
a surface of a glass positioned inside a vacuum sealed housing.
24. The method of claim 19 wherein the light source is a light
emitting diode (LED).
25. The method of claim 24 wherein the LED is configured to emit at
least one of blue light or white light.
26. The method of claim 19 wherein each of the illuminating the
photocathode, receiving at the first dynode, and receiving at the
second dynode are performed within a vacuum.
27. The method of claim 19 wherein the photocathode receives light
from the light source at a first side of the photocathode and emits
the first plurality of electrodes from a second side of the
photocathode, wherein the first side is positioned opposite the
second side.
28. The method of claim 19 further comprising using a third dynode
in series with the second dynode.
Description
CROSS-REFERENCE
[0001] The present application relies on U.S. Provisional Patent
Application No. 62/597,155, entitled "X-Ray Tomography Inspection
Systems and Methods" and filed on Dec. 11, 2017, for priority,
which is herein incorporated by reference in its entirety.
FIELD
[0002] The present specification relates to X-ray scanning systems.
More particularly, the present specification relates to a
stationary gantry X-ray inspection system having a plurality of
X-ray sources positioned around a volume of inspection such that
the sources emit X-ray beams having different beam angles.
BACKGROUND
[0003] X-ray computed tomography (CT) scanners have been used in
security screening in airports for several years. A conventional
system comprises an X-ray tube that is rotated about an axis with
an arcuate X-ray detector which is also rotated, at the same speed,
around the same axis. The conveyor belt on which the baggage is
carried is placed within a suitable aperture around the central
axis of rotation, and moved along the axis as the tube is rotated.
A fan beam of X-radiation passes from the source through the object
to be inspected and subsequently to the X-ray detector array.
[0004] The X-ray detector array records the intensity of X-rays
passed through the object to be inspected at several locations
along its length. One set of projection data is recorded at each of
a number of source angles. From these recorded X-ray intensities,
it is possible to form a tomographic (cross-sectional) image,
typically by means of a filtered back projection algorithm.
Rotational scanning of the X-ray source is applied in order to
produce an accurate tomographic image of an object, such as a bag
or package, during the longitudinal motion of the conveyor on which
the object is carried.
[0005] In a conventional X-ray scanner system, an X-ray tube
comprises an electron source that is designed to emit electrons
towards an anode which is held at high positive potential
(typically in the range 15 kV to 450 kV) with respect to the
electron source. Electrons emitted at low potential from the
electron source are accelerated in the electric field that exists
between the electron source (cathode) and the anode. When the
accelerated electrons impact on the anode surface, a fraction
(typically 1%) of their energy is emitted as X-radiation, the
balance resulting in thermal heating of the anode or electrons
backscattered from the anode.
[0006] Typically an electron source for an X-ray tube comprises a
thermionic emitter such as a heated tungsten wire. Electrons in the
wire can gain sufficient energy to "boil" off from the surface of
the wire into the surrounding vacuum from where they may be
extracted into an electric field that exists between the cathode
and anode.
[0007] Such electron sources are very commonly used, typically in a
coiled form, in a wide range of X-ray tubes. Such sources are
characterized by an operating temperature upwards of 1500K with a
filament resistance of a few ohms when at operating temperature and
an operating power dissipation in the range 1 W to 20 W depending
on the application in which the tube is designed to be used.
[0008] In advanced X-ray sources, the filament may be used to
indirectly heat a secondary electron emitting region that will
typically be coated in a dispenser cathode material (e.g. thoriated
porous tungsten) with low work function. During operation, the
temperature of the dispenser cathode material is generally
significantly lower (e.g. 1200K operating temperature) than a
standard tungsten filament (>1500K), which means that the
thermal power required to heat the dispenser cathode to operating
temperature is less than that for the standard tungsten filament
and typically in the range 0.3 W-2 W.
[0009] This lower operating power is beneficial in terms of
reducing overall X-ray tube power dissipation. This is especially
important in X-ray sources that have multiple electron guns, such
as a multi-focus X-ray tube for use in stationary gantry computed
tomography.
[0010] In some applications, power dissipation from the electron
source(s) can be detrimental to overall system design, especially
in applications where there is limited space, where there is
limited capacity for thermal heat dissipation, and where multiple
electron emitters are required in a single tube envelope, such as
in stationary gantry computed tomography systems to be deployed in
high throughput baggage screening applications.
[0011] Over the past fifty years, considerable work has been
conducted on the development of cold cathode emitters--designed to
operate at room temperature--which tend to rely on field emission
of electrons from sharp points or tips. The latest generation of
such emitters tend to take advantage of developments in
nano-engineering, such as growth of carbon nanotubes. However,
despite significant work, such electron sources still require
operation in extreme high vacuum (e.g. 10-9 Torr) and their
emission current density is limited by the electric field applied
to each emission point and the total surface area of the emission
region. In X-ray tube applications, the sensitive point-like
emission sources get damaged easily by reverse ion bombardment from
ions generated as a result of ablation of the X-ray tube target or
other gas molecules present in the vacuum. Therefore, the
development of X-ray sources based on field emission has yet to be
proven successful.
[0012] The limitations of current electron sources for use in low
operating power multi-focus X-ray sources is therefore recognized.
There is a need for an alternative X-ray source that mitigates the
reliability issues seen with field emission based X-ray sources,
and reduces operating power substantially compared to standard
thermionic electron sources.
SUMMARY
[0013] The following embodiments and aspects thereof are described
and illustrated in conjunction with systems, tools and methods,
which are meant to be exemplary and illustrative, not limiting in
scope.
[0014] In some embodiments, the present specification discloses an
optical device configured for use in an X-ray inspection system,
the optical device comprising: a light source configured to emit
light; a photocathode proximate the light source, positioned such
that it is in a path of the light emitted by the light source, and
configured to emit a first plurality of electrons; a first dynode
positioned to receive the first plurality of electrons emitted by
the photocathode and configured to emit a second plurality of
electrons in response to receiving the first plurality of
electrons; and a second dynode positioned to receive the second
plurality of electrons emitted by the first dynode and configured
to emit a third plurality of electrons in response to receiving the
second plurality of electrons.
[0015] Optionally, at least a portion of the optical device is
enclosed in a vacuum sealed housing and wherein the vacuum sealed
housing comprises at least one of glass or metal.
[0016] Optionally, the light source is positioned outside the
housing.
[0017] Optionally, the photocathode comprises a material deposited
over an optically transparent glass within the vacuum sealed
housing.
[0018] Optionally, the optical device further comprises at least
one of a grid electrode or a focus electrode.
[0019] Optionally, the light source is a light emitting diode
(LED).
[0020] Optionally, the LED emits at least one of blue light or
white light.
[0021] Optionally, the light source a LASER.
[0022] Optionally, the photocathode, the first dynode, and the
second dynode are placed within the vacuum sealed housing.
[0023] In some embodiments, the present specification discloses an
X-ray inspection system comprising: a stationary X-ray source
extending around a scanning volume, wherein the stationary X-ray
source comprises: a plurality of source points, wherein each of the
plurality of source points is configured to generate X-rays and
direct the X-rays into the scanning volume and wherein each of the
plurality of source points comprises: a light assembly comprising:
a light source configured to emit light; a photocathode proximate
the light source, positioned such that it is in a path of the light
emitted by the light source, and configured to emit a first
plurality of electrons; and a first dynode positioned to receive
the first plurality of electrons emitted by the photocathode and
configured to emit a second plurality of electrons in response to
receiving the first plurality of electrons; and a second dynode
positioned to receive the second plurality of electrons emitted by
the first dynode and configured to emit a third plurality of
electrons in response to receiving the second plurality of
electrons; and an anode assembly positioned to receive the third
plurality of electrons and configured to convert the third
plurality of electrons to said X-rays; an X-ray detector array
extending around the scanning volume and arranged to detect X-rays
from the anode assembly which have passed through the scanning
volume; a conveyor arranged to convey the items through the
scanning volume; and at least one processor for processing the
detected X-rays to produce images of items passing through the
scanning volume.
[0024] Optionally, the anode assembly emits the X-rays from a
plurality of different emission points.
[0025] Optionally, each of the plurality of source points is
enclosed in a vacuum sealed housing and wherein the housing
comprises at least one of glass or metal.
[0026] Optionally, the light assembly further comprises at least
one of a grid electrode or a focus electrode.
[0027] Optionally, the photocathode comprises a material deposited
over an optically transparent glass.
[0028] Optionally, the light source is a light emitting diode
(LED).
[0029] Optionally, the stationary X-ray source comprises the
plurality of source points and the anode assembly positioned in an
enclosed housing.
[0030] Optionally, the LED emits one of blue light or white
light.
[0031] Optionally, the photocathode and at least one of the first
dynode or the second dynode are placed inside a vacuum sealed
housing.
[0032] In some embodiments, the present specification discloses a
method for scanning items using an X-ray inspection system, the
method comprising: passing an item to be scanned through an
enclosed inspection volume; emitting X-rays from a stationary X-ray
source positioned around the inspection volume by: illuminating a
photocathode from a light source that emits light towards the
photocathode; receiving, at a first dynode, a first plurality of
electrons emitted by the photocathode; receiving, at a second
dynode, a second plurality of electrons emitted by the first
dynode; receiving, at an anode assembly, a third plurality of
electrons emitted by the second dynode and converting the third
plurality of electrons to said X-rays; detecting X-rays from the
stationary X-ray source which have passed through the enclosed
inspection volume; and processing the detected X-rays to produce
scanning images of the items.
[0033] Optionally, a number of the second plurality of electrons is
greater than a number of the first plurality of electrons.
[0034] Optionally, a number of the third plurality of electrons is
greater than a number of the second plurality of electrons.
[0035] Optionally, at least one of the photocathode, first dynode,
or second dynode is enclosed in a vacuum sealed glass or metal
housing.
[0036] Optionally, the photocathode is deposited on a surface of a
glass positioned inside a vacuum sealed housing.
[0037] Optionally, the light source is a light emitting diode
(LED).
[0038] Optionally, the LED is configured to emit at least one of
blue light or white light.
[0039] Optionally, each of the steps of illuminating the
photocathode, receiving at the first dynode, and receiving at the
second dynode are performed within a vacuum.
[0040] Optionally, the photocathode receives light from the light
source at a first side of the photocathode and emits the first
plurality of electrodes from a second side of the photocathode,
wherein the first side is positioned opposite the second side.
[0041] Optionally, the method further comprises using a third
dynode in series with the second dynode.
[0042] In some embodiments, the present specification discloses a
light assembly for use in an X-ray inspection system for scanning
items, the assembly comprising: a light source; and at least two
dynodes, wherein a first dynode is positioned to receive electrons
emitted by the photocathode and a second dynode is positioned to
receive electrons emitted by the first dynode.
[0043] Optionally, the assembly is enclosed in a glass or metal
envelope.
[0044] Optionally, the light source is positioned outside the glass
or metal envelope.
[0045] Optionally, the assembly further comprises at least one of:
a grid and a focus electrode.
[0046] Optionally, the light source is a light emitting diode
(LED).
[0047] Optionally, the LED emits one of blue light or white
light.
[0048] Optionally, the light source a LASER.
[0049] Optionally, the at least two dynodes are placed inside a
vacuum housing.
[0050] In some embodiments, the present specification discloses a
method for scanning items using an X-ray inspection system, the
method comprising: emitting X-rays from a stationary X-ray source
comprising at least one cathode assembly from which X-rays can be
directed through the scanning volume, wherein the emitting from the
cathode assembly comprises: emitting light from a light source and
directing said light towards a first dynode; generating electrons,
using said light, by the first dynode; emitting generated electrons
by the first dynode towards a second dynode; and receiving at the
second dynode, electrons emitted by the first dynode, wherein the
electrons are multiplied and emitted further by the second dynode;
converting emitted electrons to X-rays by an anode assembly;
detecting X-rays from the anode assembly which have passed through
the scanning volume; and processing the detected X-rays to produce
scanning images of the items.
[0051] Optionally, the cathode assembly is enclosed in a glass or
metal envelope.
[0052] Optionally, the emitting light from a light source comprises
emitting light from a light emitting diode (LED). Optionally, the
LED emits one of blue light or white light.
[0053] Optionally, the steps of generating electrons by the first
dynode; emitting generated electrons by the first dynode towards
the second dynode; and receiving at the second dynode, electrons
emitted by the first dynode, wherein the electrons are multiplied
and emitted further by the second dynode are performed with a
vacuum.
[0054] Optionally, the method further comprises using at least one
more dynode in succession with the second dynode, wherein each
dynode multiplies received electrons.
[0055] The aforementioned and other embodiments of the present
specification shall be described in greater depth in the drawings
and detailed description provided below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0056] These and other features and advantages of the present
invention will be further appreciated, as they become better
understood by reference to the detailed description when considered
in connection with the accompanying drawings:
[0057] FIG. 1A is a perspective view of a conventional X-ray
inspection system;
[0058] FIG. 1B is a schematic diagram illustrating a plurality of
views of the scanning unit of FIG. 1A;
[0059] FIG. 2A illustrates an X-ray tube cathode assembly, in
accordance with some embodiments of the present specification;
[0060] FIG. 2B shows a circuit diagram for operation of a
photocathode-based electron source shown in FIG. 2A, in accordance
with some embodiments of the present specification;
[0061] FIG. 3 is a flow chart illustrating some of the exemplary
steps of a method for scanning items using an X-ray inspection
system, in accordance with embodiments of the present
specification;
[0062] FIG. 4 is a flow chart illustrating some of the exemplary
steps of another method for scanning items using an X-ray
inspection system, in accordance with some embodiments of the
present specification;
[0063] FIG. 5 is a flow chart of a plurality of exemplary steps of
a method of manufacturing the X-ray source or electron gun of FIG.
2A; and
[0064] FIG. 6 illustrates a variation in photocathode radiation
sensitivity at different wavelengths of light.
DETAILED DESCRIPTION
[0065] It is understood by one of ordinary skill in the art that
photomultiplier tubes are commonly used in the field of radiation
detection to take the flash of light caused by radiation
interaction in a scintillation detector (such as sodium iodide,
NaI), to convert this to electrons using a photocathode and to then
accelerate the generated electrons through an electric field to a
first dynode whereby each interacting electron from the
photocathode is absorbed and re-emitted in the form of several
electrons, each of which is then accelerated towards a second
dynode. The process repeats at the second and subsequent dynodes
until a large signal has been produced at the final dynode and the
resulting amplified signal is recorded at an anode. The whole
photomultiplier tube is operated in a standard vacuum, typically in
the range 10.sup.-7 to 10.sup.-6 Torr. Typically a gain in the
range of n=5 to 20 is achieved at each dynode. For a system with 10
dynodes, this means a gain of n.sup.10 for the photomultiplier as a
whole.
[0066] Embodiments of the present specification recognize that the
photocathode material (for example, Caesium Iodide (CsI)) absorbs
optical photons within its bulk which releases an electron into the
conduction band through the photoelectric effect. This conduction
electron is free to migrate throughout the bulk of the
photocathode. If it gets to the surface of the material, it has a
good probability of escaping from the photocathode material into
the vacuum. Therefore, this is a bulk effect, not a point emission
effect, which is controlled by the thickness of the photocathode
material, the wavelength of light entering the photocathode and the
strength of the electric field at the emission surface of the
photocathode. In these respects a photocathode is an ideal source
of electrons for an X-ray tube cathode. The emission of the
electron source may be therefore switched on and off by modulating
the intensity of the applied optical beam at the photocathode. This
is a key characteristic of a multi-focus X-ray source in which it
is necessary to turn each one of the multiple electron sources
within a single X-ray tube on and off with high temporal precision
and repeatability.
[0067] In embodiments, the present specification provides an
inspection system with multiple source points used to scan the
scanning volume. In an embodiment, the inspection system is a
real-time tomography (RTT) system. In an embodiment, the source
points are arranged in a non-circular or substantially rectangular
geometry around the scanning volume. In embodiments, the inspection
system is cost effective, has a smaller footprint and may be
operated using regular line voltage to supply power to the high
voltage power supply, which is then used to provide power to the
X-ray source.
[0068] In various embodiments, the X-ray sources emit fan beams
which have different emission points based on the location of the
X-ray source points with respect to the imaging volume.
[0069] It should be noted that the systems described throughout
this specification comprise at least one processor to control the
operation of the system and its components. It should further be
appreciated that the at least one processor is capable of
processing programmatic instructions, has a memory capable of
storing programmatic instructions, and employs software comprised
of a plurality of programmatic instructions for performing the
processes described herein. In one embodiment, the at least one
processor is a computing device capable of receiving, executing,
and transmitting a plurality of programmatic instructions stored on
a volatile or non-volatile computer readable medium.
[0070] The present specification is directed towards multiple
embodiments. The following disclosure is provided in order to
enable a person having ordinary skill in the art to practice the
invention. Language used in this specification should not be
interpreted as a general disavowal of any one specific embodiment
or used to limit the claims beyond the meaning of the terms used
therein. The general principles defined herein may be applied to
other embodiments and applications without departing from the
spirit and scope of the invention. Also, the terminology and
phraseology used is for the purpose of describing exemplary
embodiments and should not be considered limiting. Thus, the
present specification is to be accorded the widest scope
encompassing numerous alternatives, modifications and equivalents
consistent with the principles and features disclosed. For purpose
of clarity, details relating to technical material that is known in
the technical fields related to the invention have not been
described in detail so as not to unnecessarily obscure the present
specification.
[0071] In the description and claims of the application, each of
the words "comprise" "include" and "have", and forms thereof, are
not necessarily limited to members in a list with which the words
may be associated. It should be noted herein that any feature or
component described in association with a specific embodiment may
be used and implemented with any other embodiment unless clearly
indicated otherwise.
[0072] For the purposes of this specification, a filtered
back-projection method is defined to describe any transmission or
diffraction tomographic technique for the partial or complete
reconstruction of an object where a filtered projection is
back-projected into the object space; i.e., is propagated back into
object space according to an inverse or approximate inverse of the
manner in which the beam was originally transmitted or diffracted.
The filtered back-projection method is usually implemented in the
form of a convolution of filters and directly calculates the image
in a single reconstruction step.
[0073] For the purposes of this specification an iterative
reconstruction method refers to iterative algorithms (versus a
single reconstruction algorithm) used to reconstruct 2D and 3D
images such as a computed tomography where an image must be
reconstructed from projections of an object.
[0074] For the purposes of this specification, thermionic emission
refers to a thermally induced charge emission process. Thermal
energy drives charge carriers over a potential energy barrier,
thereby causing an electric current. In conventional X-ray tubes,
the charge carriers are electrons emitted from heated cathodes.
[0075] For the purposes of this specification, a photocathode is a
negatively charged electrode which is coated with a photosensitive
compound. The photocathode emits electrons when illuminated,
thereby causing an electric current. Photocathodes typically
operate in a vacuum.
[0076] FIG. 1A is a perspective view of a conventional scanning
unit 100, shown from a first side 145, comprising a substantially
rectangular housing/enclosure 101 for housing a plurality of X-ray
source points and detectors. The source points, which are
positioned at the surface of an anode, are located in a first plane
normal to the plane of a conveyor. The detectors are located in a
plane parallel to the source points and are also normal to the
plane of a conveyor but offset from the source point plane by a
distance ranging from 1 mm to 20 mm. In embodiments, there are
between 100 and 1000 source points arranged around the perimeter of
the tunnel in a rectangular, circular or other similar shape. It
should be appreciated that, in alternate embodiments, the housing
101 may have a circular, or a quadrilateral shape, such as, but not
limited to, a square, or any other shape. An object under
inspection is conveyed through a first open end or scanning
aperture 103, enters the inspection region, and exits through a
second open end (opposite to the first open end 103). In accordance
with an embodiment, both feed and return conveyor loops pass
through a space 116 just below the inspection region 106, while
space or compartment 140 is reserved in the base of the scanning
system (approximately 200 mm deep) to accommodate automated return
of trays when integrated with an automatic tray return handling
system.
[0077] FIG. 1B illustrates a plurality of views of an exemplary
scanning unit 100 of FIG. 1A. The scanning unit 100 may be designed
for reduced power usage and reduced noise. Referring now to FIG.
1B, view 141 illustrates a first open end or scanning aperture 103
of the scanning system 100 for objects under inspection to enter
the inspection region. In some embodiments, the scanning aperture
103 and thus, inspection volume, has a width of 620 mm and a height
of 420 mm. View 142 is a side view (as seen from first side 145 of
FIG. 1A) along a longitudinal direction of the scanning unit 100.
View 143 is a top view along the longitudinal direction of scanning
unit 100. It should be noted that the longitudinal length of
scanning system 100 as shown in view 143 is to accommodate for
higher levels of X-ray scatter from the object under inspection
which is caused by the higher beam current that is necessarily used
to produce a clear image. Views 141, 142 also illustrate the space
140 through which tray can pass when integrated with an automatic
tray return handling system.
[0078] While FIGS. 1A and 1B illustrate exemplary scanning systems
for implementing the various embodiments of the present
specification, other scanning systems may also incorporate these
embodiments. In embodiments, the present application relates to
U.S. Pat. No. 8,085,897 entitled "X-Ray Scanning System", and
issued on Dec. 27, 2011, and its family members. The present
application also relates to the scanning system described in U.S.
Pat. No. 7,876,879, entitled "X-Ray Tomography Inspection Systems",
and issued on Jan. 25, 2011, and its family members. Embodiments of
the electron source of the present specification may also be
applicable to the X-ray tube described in U.S. Pat. No. 8,824,637,
entitled "X-Ray Tubes", and issued on Sep. 2, 2014. Embodiments of
the electron source of the present specification may also be
applicable to the electron gun described in U.S. Pat. No.
9,618,648, entitled "X-Ray Scanners", and issued on Apr. 11, 2017.
Embodiments of the electron source of the present specification may
also be applicable to the X-ray scanner described in U.S. Pat. No.
8,243,876, entitled "X-ray Scanners", and issued on Aug. 14, 2012.
Embodiments of the electron source of the present specification may
also be applicable to the X-ray scanner described in U.S. Pat. No.
7,949,101, entitled "X-Ray Scanners and X-Ray Sources Therefor",
and issued on May 24, 2011. All of the above-mentioned applications
are herein incorporated by reference.
[0079] FIG. 2A illustrates an X-ray tube cathode assembly 200, in
accordance with some embodiments of the present specification. In
embodiments, assembly 200 is placed inside a vacuum housing 218,
and comprises an optically transparent window through which a beam
of light is passed from a light source. In embodiments, the
wavelength of emission of diode 202 is matched with the wavelength
of maximum emissivity of a photocathode material. The wavelength of
light emitted by the light source such as diode 202 is chosen such
that optical photons have sufficient energy to excite a valence
electron into the conduction band of the photocathode material. In
embodiments, this may mean operation in the blue to green
wavelength range depending on materials chosen. In one embodiment,
the light source is a light emitting diode (LED) 202, which,
optionally, has a high brightness. In accordance with embodiments
of the present specification, brightness is defined in terms of
power (Watts (W)) or luminosity (milliCandela (mCd)). A high
brightness LED has power in the range of 0.5 W to 2 W with
luminosity in the range of 500 mCd to 8000 mCd depending on the
frequency (for example, typically blue wavelengths will have lower
luminosity than green wavelengths). In embodiments, the brightness
and luminosity are dependent upon many factors, including but not
limited to required beam current, photocathode material, and
wavelength of excitation so it should be noted that these
parameters may be adjusted depending on the requirements of the
overall system of the present specification.
[0080] In embodiments, the photocathode material chosen is at least
one of CsI, CsSb, and GaAsP. FIG. 6 illustrates a graph 600 showing
a variation in photocathode radiation sensitivity at different
wavelengths of light. In embodiments, less than 0.2 mA is required
from the photocathode, which may be obtained from a 10 mW LED
brightness source, assuming 100% conversion efficiency from power
to light and 100% Quantum Efficiency (QE). In reality, the QE of
the photocathode is around 10%, which implies that only one in ten
arriving optical photons create a photoelectron from the
photocathode. Therefore, in embodiments, an optical power ranging
from 50 mW to 300 mW is used for LED brightness in the present
specification. In an embodiment, an optical power of 100 mW is used
for the LED brightness in the present specification. The brightness
of diode 202 is chosen such that the diode is capable of delivering
the requisite photocathode current. In another embodiment, the
light source is a LASER, or any other light source permissible by
the implementations of the present specification, which provides
sufficient brightness to operate a photocathode. In embodiments,
the optically transparent window is fitted with optically
transparent glass in the wavelength range of interest (typically a
silica or quartz glass 210). The glass is selected because it is
transparent and efficient at transmitting white light or blue
light. The glass 210 is also used to close or seal the vacuum
envelope.
[0081] In embodiments, the photocathode is placed in proximity to
diode 202. In some embodiments, the photocathode is placed within a
range of 0.1 mm to 5 mm from diode 202. Diode 202 is used to excite
one photocathode. In an embodiment of a multi-focus X-ray tube
comprising multiple photocathodes, each photocathode is associated
with one photodiode, similar to diode 202. In some embodiments,
electron sources arrays are built in multiples of eight (8) or
sixteen (16) in which case, there are 8 or 16 individual LED's
irradiating 8 or 16 individual photocathode/dynode assemblies.
Light emitting diode 202 is used to commence or "kick-off" a
cascade of electron generation. Light emitting diode 202 operates
at a ground potential and creates a pulsed light output. In
embodiments, each light emitting diode 202 operates outside of the
vacuum envelope so that they can be easily replaced. This optical
beam generated by each light emitting diode 202 is absorbed by a
photocathode 204 which is held at a fixed electrical potential by a
transparent metal film (for example indium tin oxide, ITO). In
embodiments, photocathode 204 has properties including, but not
limited to, the ability to provide efficient light absorption;
optimal conversion of optical energy to emitted electrons (good
emissivity); stability under high light irradiation conditions; and
a low work function.
[0082] In an embodiment, the material employed for photocathode 204
is evaporated onto glass 210 when heated under vacuum. In one
embodiment, photocathode 204 is heated when the tube is baked
during subsequent manufacturing using a vacuum oven. Photocathode
material 204 may be placed on the vacuum side of glass 210 as a
small drop or pellet. In other words, photocathode 204 coats glass
210 on the side of the glass that is opposite to the side facing
light emitting diode 202 (the vacuum side of glass 210).
[0083] In embodiments, distance between first dynode 206 and
photocathode 204 ranges from 2 mm to 50 mm. In one embodiment, a
first dynode 206 is placed at a distance of 5 mm from photocathode
204. First dynode 206, placed in the vacuum across photocathode
204, is at a negative potential, however, its negative potential is
less than that of photocathode 204. The angle between photocathode
204 and first dynode 206 is engineered to provide a relatively
uniform field across the surface of photocathode 204 and first
dynode 206. In some embodiments, a potential difference of 30V to
400V, and preferably 50V to 200V, is maintained between
photocathode 204 and first dynode 206 and between subsequent
dynodes. Dynode 206, and any other dynode are shaped liked arcs, in
accordance with some embodiments of the present specification. The
shape of the dynodes is designed to obtain a relatively uniform
electric field at the surface of each dynode to achieve a stable
gain from each dynode. The dynode shape is also designed to create
an electric field from the photocathode to first dynode 206 and
then again from first dynode 206 to the second dynode 208 and the
next dynode and so forth. In some other embodiments, each dynode
has any other shape that is able to perform the functions
prescribed in the embodiments of this specification. Electrons
emitted from photocathode 204 are accelerated in an electric field
to a first dynode 206 which multiplies each arriving electron by a
factor of typically 5 to 20. The multiplication factor depends on
several factors including dynode material and the energy of the
arriving electrons. The angle between first dynode 206 and a second
dynode 208, and similarly between each consecutive dynode, is
engineered to provide relatively uniform field across their
surfaces. In embodiments, the angle between first dynode 206 and
second dynode 208 ranges from 0 degrees (measured from the bottom
left of the first dynode 206 to the bottom left of the second
dynode 208) to 90 degrees (measured from the top right of the first
dynode 206 to the bottom left of the second dynode 208). In
embodiments, the dynodes are all positioned in a single plane. In
one embodiment, each dynode, including dynode 206, is coated with a
simple metal or a low electron affinity coating such as Cesium
Antimony or an antimony-tin alloy. In an embodiment, the
photocathode metal that is employed may be characterized by a low
vacuum work function.
[0084] Electrons emitted from first dynode 206 are accelerated to
second dynode 208 where a further multiplication occurs. In
embodiments, distance between first dynode 206 and second dynode
208 ranges from 2 mm to 20 mm. Second dynode 208, placed in the
vacuum across first dynode 206, is at a negative potential,
however, its negative potential is less than that of first dynode
206. In embodiments, arriving electron energy at dynode 208 is
approximately in the range of 50 eV to 200 eV. In embodiments, the
arriving energy is proportional to the accelerating voltage, which
may be in a range of 30V to 400V, and preferably 50V to 200V. The
energy of the arriving electrons is driven by the applied potential
difference and the distance between the various parts of the dynode
surfaces. The electrons that hit dynode 208 are further multiplied
by a factor of 5 to 20. As a result, there is a multiplicative gain
of electrons. Overall, a gain of 25 to 400 is achieved through the
two dynode stages depending on photocathode 204 and the
optimization, orientation, geometry, and applied voltages of first
dynode 206 and second dynode 208. The preferred configuration of
the photocathodes and dynodes relative to one another is such that
the electric field is equalized over the surface of dynodes 206 and
208 and photocathode 204 as far as possible since the probability
of an electron escaping into the vacuum between dynodes is driven
by the electric field at the dynode surface, which, in turn,
depends on the dynode geometry.
[0085] In some embodiments, more than two dynodes are deployed to
increase the multiplicative gain of electrons. The multiplicative
gain stages result in decoupling the stage of electron creation
from the process of electron generation for forming an X-ray beam.
As a result, the first source of electrons, in this case
photocathode 204 is operated with a light source enabling lower
temperatures for operating the cathode.
[0086] In some embodiments, the photocathode 204 may be removed and
the light from the external light source may irradiate the first
dynode directly in order to generate secondary electron
emission.
[0087] In some embodiments, electrons emitted in an electron beam
212 from second dynode 208 are extracted into an electron focusing
structure to direct the generated electrons of beam 212 to an X-ray
anode 214. In embodiments, the electron focusing structure is focus
electrode or grid electrode to shape the electric field around the
electron emitter for controlling the beam cross-section so that a
suitable focal spot is formed on the anode target. In some
embodiments, the focus electrode is formed from a refractory
material, such as tungsten or molybdenum, in order to survive high
energy ion bombardment of ions liberated from residual gas atoms or
atoms ablated from the target. The focus electrode may be open, or
may be in a mesh form with multiple apertures, including a simple
cruciform grid.
[0088] In embodiments, a focusing electrode is an electrode to
which a potential is applied to control the cross-sectional area of
the electron beam in an X-ray tube. In some embodiments, the
focusing optic(s) is in a cylindrical form which focuses the
electron beam from the final dynode in both lateral and
longitudinal (X-Y) directions at once. This approach may be suited
to a compact electron source design with similar X-Y dimensions at
the electron emitter, such as a button source. In some alternative
embodiments, separate linear focusing structures are used to focus
independently in X and Y directions. The independent focusing
approach may be suited to electron sources which are extended in
one direction compared to the other, such as a line source. In
embodiments, an electron focusing structure 216 and any
electrostatic grid (not shown) that is placed above second dynode
208 is held at ground potential to act as the primary point of
discharge for any high voltage breakdown that may occur during tube
conditioning as part of tube manufacture or at any point thereafter
during system operation.
[0089] As shown in FIG. 2A, dynodes 206, 208 may be configured to
introduce a spatial offset between photocathode 204 and the point
of emission of electron beam 212. In embodiments, the spatial
offset may be defined as the distance (in the horizontal direction)
between the vertical axes that pass through centers of each dynode
206 and 208. This offset ensures that any reverse ion bombardment
affects electron focusing structure 216 and final dynode only and
does not affect photocathode 204 or first dynode 206. This helps to
ensure long tube lifetime even in relatively poor vacuum
operation.
[0090] In embodiments, an electron gun operates at a beam current
ranging from 2 mA to 50 mA, and preferably ranging from 4 mA to 20
mA. In one embodiment, the electron gun operates at 4 mA,
preferably for small tunnel checkpoint applications. In one
embodiment, the electron gun is used to operate at 20 mA,
preferably for large tunnel hold baggage screening applications. A
beam current of 20 mA equates to generation of approximately
10.sup.15 electrons per second at the surface of photocathode 204,
or photocathode current of 800 .mu.A, given dynode gains of 5. This
photocathode current reduces to 50 .mu.A for dynode gain of 20.
These currents are achieved using off-the-shelf, low cost, high
brightness, options for light emitting diode 202.
[0091] FIG. 2B shows a circuit diagram for operation of a
photocathode 204-based electron source shown in FIG. 2A, in
accordance with some embodiments of the present specification. In
embodiments, photocathode (PC) 204 is illuminated by light emitting
diode (LED) 202. Electrons from PC 204 are firstly accelerated to
Dynode 1 (Dy1) 206 and then to Dynode 2 (Dy2) 208. The dynodes from
Dy2 208 then accelerate towards an optional perforated grid (G1)
420 and then to a final cathode (K) 222. Electrons that pass from
cathode 222 into the main X-ray vacuum envelope then accelerate to
X-ray anode (A) 214.
[0092] In some embodiments, grid G1 220 is introduced to cathode
assembly 200. The effect of G1 220 is to create a controlled field
region between G1 220 and cathode, K 222. This allows beam current
to be modulated by adjusting the potential on G1 220 relative to K
222. Switching G1 220 to a positive potential with respect to K 222
switches the electron beam off completely regardless of the
illumination of LED 202. Thus, G1 220 provides a second level of
control of the emitted electron beam, including a wider on-to-off
beam current ratio plus controllable beam current by setting the
potential of G1 220 relative to K 222. In some embodiments, one
grid is provided for each emitter in order to eliminate
cross-talk.
[0093] In an alternative embodiment, assembly 200 is constructed
without grid G1 220. In this embodiment, electron emission is
controlled solely by the switching of LED 202. Referring
simultaneously to FIGS. 2A and 2B, when LED 202 is switched on and
illuminating photocathode 204, an electron beam 212 is produced and
this is accelerated into the main vacuum envelope to produce an
X-ray beam 224. When LED 202 is switched off, no photocathode
electrons are generated and no X-ray beam is produced. Therefore,
X-ray emission is controlled directly by modelling the brightness
of LED 202. Intensity of LED 202 is directly proportional to the
X-ray tube beam current.
[0094] In one embodiment, multiple photocathodes, dynodes and
cathodes are connected to a single set of potentials that are
supplied to the electron source components via a single set of
electrical vacuum feedthroughs, one set of feedthroughs per X-ray
tube. This is very efficient compared to thermionic sources where
multiple electrical feedthroughs are required to provide power to
the filaments and to control the respective grid signals. The
present embodiments do not include any thermionic element and so
the electron source operates at zero power other than when actively
generating an X-ray beam.
[0095] In an alternative embodiment, a field emission source is
used for electron generation. In an embodiment, the field emission
source and focusing optics similar to the configuration described
in context of FIG. 2B are used for electron generation. Cathode and
grid materials for a cathode in a field emission source may employ
a highly refractory material such as tungsten or molybdenum. The
electric field (E) formed is due to an electrical potential (V)
applied between the grid and cathode and the distance (d) between
them (E=V/d). In embodiments, the field emission source is placed
in a closer proximity to the grid, relative to the proximity of LED
202 and grid G1 220.
[0096] Embodiments of cathode assembly 200 can be implemented in
various embodiments of an X-ray inspection system. Cathode assembly
200 may be a component of an X-ray source point, where multiple
X-ray source points form an X-ray screening system. Alternatively,
a single source point can be used in a standard single focus X-ray
source. Alternatively, two source points can be used in a dual
focus source to create broad (high power) and fine (low power)
focus, for example. Embodiments of the present specification
provide an advantage over conventional cathode assemblies as the
cathode temperatures of the present embodiments can be kept at room
temperature, whereas heated cathodes in conventional cathode
assemblies operate at above 1500K.
[0097] FIG. 3 is a flow chart illustrating some of the exemplary
steps of a method for scanning items using an X-ray inspection
system, in accordance with embodiments of the present
specification. In embodiments, a stationary X-ray source is formed
by multiple X-ray source points that surround a scanning volume
that may be of arbitrary shape (generally rectangular or circular).
The X-ray source points are positioned at the surface of anode are
located in a first plane normal to the plane of a conveyor while
the detectors are located in a plane parallel to the source points
and are also normal to the conveyor but offset from the source
point plane by a distance that ranges from 1 mm to 20 mm. In
embodiments, there are between 100 and 1000 source points arranged
around the perimeter of the tunnel in a rectangular, circular or
other similar shape. Each source point includes a cathode assembly,
such as the one described in context of FIGS. 2A and 2B. At 302, a
photocathode is lit from a first side using a light source. In
embodiments, the light source is an LED that emits white light. In
embodiments, the light source is an LED that emits blue light. It
should be noted herein that the responsivity of the photocathode is
generally over a broad spectral range and that white LEDs are
generally brighter than blue LEDs, although either may be employed.
At 304, lighting the photocathode causes the photocathode to emit
electrons from a second side, which is opposite to the first side.
In embodiments, the photocathode is positioned or deposited over an
optically transparent glass. At 306, a first dynode receives the
electrons emitted by the photocathode. The first dynode multiplies
received electrons and emits them towards a second dynode. At 308,
the second dynode receives the electrons from the first dynode and
further multiplies them and emits the multiplied electrons. In some
embodiments, the system includes at least one or more additional
dynodes in succession after the second dynode. Each dynode is
configured to receive electrons from the previous dynode and
further multiply electrons. In an alternate embodiment, the light
source irradiates the first dynode directly without the use of a
separate photocathode and electrons emitted from the first dynode
are directed to a second and subsequent dynode(s). In yet another
alternative embodiment, electrons are generated with a field
electron emission source instead of a light source. In embodiments,
steps from 304 to 308 are performed in a vacuum. In embodiments, a
glass or metal enclosure is used to perform these steps. At step
309, an anode assembly receives the electron generated by the
dynodes and converts them to X-rays. Subsequently, at 310, the
X-rays from the anode assembly that have passed through the
scanning volume are detected by one or more detectors. At 312, the
detected X-rays are processed by a processor in order to produce
scanning images of the items being scanned.
[0098] FIG. 4 is a flow chart illustrating some of the exemplary
steps of another method for scanning items, in accordance with some
embodiments of the present specification. In an embodiment, as
described with respect to FIG. 4, a light source irradiates the
first dynode directly without the use of a separate photocathode.
Electrons emitted from the first dynode are directed to a second
dynode and optionally, subsequent dynodes. At step 402, a light
source emits light towards a first dynode. Each source point
includes a light source assembly. In embodiments, the light source
is an LED, or a LASER. At step 404, the first dynode receives the
light from the light source and thereby generates electrons. At
step 406, the first dynode emits the generated electrons and
directs them towards a second dynode. At 408, the second dynode
receives the electrons from the first dynode, multiplies them and
subsequently, emits the multiplied electrons. In some optional
embodiments, the system includes at least one or more additional
dynodes in succession after the second dynode. Each dynode is
configured to receive electrons from the previous dynode and
further multiply electrons. In some embodiments, the light source
assembly is configured similar to the system illustrated in the
context of FIGS. 2A and 2B, without the photocathode. In
embodiments, steps 404 through 408 are performed in a vacuum. In
embodiments, a glass or metal enclosure is used to create the
vacuum. At step 409, an anode assembly converts the electrons
generated by the dynodes to X-rays. Subsequently, at step 410, the
X-rays from the anode assembly that are transmitted through the
scanning volume are detected by one or more detectors. At step 412,
the detected X-rays are processed by a processor in order to
produce images of the items being scanned.
[0099] Referring back to FIG. 1A, it should be appreciated that in
contrast to conventional rotating gantry systems, the firing
pattern for the multi-focus X-ray source 102 is not constrained to
move in a standard helical rotation about a baggage under
inspection. Thus, in various embodiments, the source firing pattern
may be fixed or random with uniform or non-uniform dwell time at
each source point 120. In various embodiments, the dwell time
ranges from 50 .mu.s to 500 .mu.s per scan projection. In some
embodiments, the dwell time is 200 .mu.s per scan projection.
[0100] In various embodiments, in order to determine substantially
accurate measures for Z-Effective and Density in the reconstructed
RTT images, both sinogram data (the multi-energy "raw" data
produced by the X-ray detectors for each source projection) and the
reconstructed image data from one or more multi-energy bins is used
in determining threat type for each object segmented from the 3D
image data. In embodiments, the reconstructed image is available as
soon as the trailing edge of a conveyor tray leaves the RTT imaging
region of the scanning unit 100.
[0101] In accordance with some embodiments, the scanning unit 100
is configured to achieve reconstructed image voxels of 0.8
mm.times.0.8 mm.times.0.8 mm over an inspection tunnel size of 620
mm wide.times.420 mm. This is equivalent to a slice image size of
775 pixels (width).times.525 pixels (height). For a conveyor tray
length of 0.8 m, there will be 1,000 slices in each 3D image. In
some embodiments, the RTT system spatial resolution is 1.0 mm at
the center of the inspection tunnel. In embodiments, the RTT system
is configured to achieve Z-effective resolution of +/-0.2 atomic
numbers with density resolution at the center of the inspection
tunnel of +/-0.5%.
[0102] FIG. 5 is a flow chart of a plurality of exemplary steps of
a method of manufacturing the X-ray source or electron gun of FIG.
2A. At step 505, the anode and cathode of the X-ray source are
machine built. At step 510, the anode section is installed into a
glass or metal top. At step 515, a slip coupling block is provided
atop the anode and a shield electrode to account for thermal
expansion. At step 520, the slip coupling block is attached to a
feed-through thermally conductive element to enable heat
dissipation from the anode. Next, at step 525, the cathode section
is installed into a glass or metal base. Finally, at step 530, the
base is sealed with the glass or metal top by using glass melting
or metal welding techniques, thereby resulting in the anode and the
cathode being enveloped in a glass or metal vacuum envelope.
[0103] The above examples are merely illustrative of the many
applications of the system of present specification. Although only
a few embodiments of the present invention have been described
herein, it should be understood that the present invention might be
embodied in many other specific forms without departing from the
spirit or scope of the invention. Therefore, the present examples
and embodiments are to be considered as illustrative and not
restrictive, and the invention may be modified within the scope of
the appended claims.
* * * * *